But I install very few apps so my experience has been from others that install so many.
Most of the time it's clear such as the adverts start popping or they "rooted" their iPhone or Android so they could install cracked apps. In those cases so many signs such as over the top adverts to being inundated with spam calls.
or in any other way detrimental to your info security or simply, your device?
What are they? And how did you find out?